1. Field of the Invention
This invention relates generally to an electro-acoustic transducer and, more particularly to a loudspeaker having an improved structure.
2. Description of the Prior Art
In recent years, various types of transducers utilizing drive systems have been developed for the purpose of reproducing high fidelity sounds.
As an example of the transducers, there is proposed a so-called ribbon type transducer. FIG. 1 is a perspective view thereof. In this example, there are provided magnets 1 and 2 disposed with opposite polarity, and a magnetic circuit is established by the magnets 1, 2, a back plate 3, and magnetic pole plates 4, 5. A ribbon type vibrating diaphragm 6 made of, for example, an aluminum foil is disposed between the magnetic pole plates 4 and 5. In FIG. 1, reference numerals 7 and 8 indicate fixing pieces, and 9 and 10 lead wires connected to the diaphragm 6 made of the metal ribbon.
In the example of FIG. 1, when a sound electric signal is applied across the diaphragm 6 through the lead wires 9 and 10, the signal current flows therethrough in the direction perpendicular to the magnetic field formed by the magnetic pole plates 4 and 5. Thus, the drive power proportional to the current is given to the metal ribbon diaphragm 6 in the vertical direction, and hence the electric signal is converted into an acoustic signal. While, when an acoustic signal is applied to the metal ribbon diaphragm 6, this diaphragm 6 is moved or vibrated in the magnetic field and hence the current proportional to the movement flows through the diaphragm 6 i.e. the acoustic signal is converted into the electric signal.
In the transducer of FIG. 1, if the metal ribbon diaphragm 6 is made of a metal foil such as an aluminum, since the impedance of the metal foil is very low such as about 0.1.OMEGA., an output impedance 8.OMEGA. of an ordinary amplifier can not be applied thereto as it is. Thus, for example, a matching transformer 11 is necessary.
Further, there has been proposed another electro-acoustic transducer as shown in FIG. 2 which is a cross-sectional view thereof and FIG. 3 which is a perspective view thereof. In this example, for example, on a plastic film 12 there is provided a spiral conductor 13 to form a vibrating diaphragm 14. At the center thereof there is located a magnet 15 which has a center pole 16 of a triangular shape in cross-section on its one pole (N-pole) and a yoke 17 of a U-shape opened upwards on the other pole (S-pole). Upper plates 18 and 19 are provided on the upper end edges of the yoke 17 in opposed relation at both sides of the center pole 16, and the diaphragm 14 is stretched between the upper plates 18 and 19 with the center portion of the diaphragm 14 being fixed to the center pole 16.
According to the prior art example of FIGS. 2 and 3, magnetic fields opposite in polarity are established between the upper plate 18 and the center pole 16 and between the upper plate 19 and the center pole 16, respectively. Because the conductor 13 of the diaphragm 14 is spiral, when an acoustic signal current is applied between both ends of the conductor 13 through the lead wires 9 and 10, the signal current flowing through the spiral conductor 13 is opposite in direction at the left and right sides of the center of diaphragm 14. As a result, the film 12 is vibrated in the same direction at the left and right sides of the center thereof and hence the electric signal is converted into an acoustic signal. By the reverse of the process, when an acoustic signal is applied to the diaphragm 14, it is converted into an electric signal. In this latter case, a sufficiently high impedance can be achieved by selecting a sufficiently long length of the conductor 13.
According to the transducer of FIGS. 2 and 3, however, since the diaphragm 14 is fixed to the center pole 16 and upper plates 18 and 19, the effective vibrating area of the diaphragm 14 becomes small and hence the frequency characteristics in the low frequency band can not be expanded. Further, the magnetic fields are established between the center pole 16 and upper plates 18, 19 but, as shown in FIG. 2, almost all of the fluxes from the center pole 16 to the plates 18 and 19 do not pass through the diaphragm 14 (refer to .PHI.A in FIG. 2) or are a so-called ineffective flux. In fact, the flux passing through the diaphragm 14 (refer to .PHI.B in FIG. 2) gives a drive force to the conductor 13, but this drive flux is a so-called leakage flux. As set forth above, the transducer of FIGS. 2 and 3 produces the drive force by the leakage flux, so that its utilization factor is low. In other words, if a magnet with great magnetomotive force is not used, a transducer having a necessary output cannot be provided. Thus, a large and expensive magnet such as an Alnic (Trade Name) magnet is necessary and hence the cost of the transducer becomes high.
Though not shown, such a transducer is also proposed in which a first magnetic plate having a number of magnetic pole pieces and a second magnetic plate having a number of magnetic pole pieces are located parallel with each other and a vibrating diaphragm having a conductor pattern is located between the first and second magnetic plates. In this transducer, the flux passes through the conductor parallel or slant thereto to produce the drive force. Further, since the diaphragm can be increased in effective vibration area, this transducer can reproduce a frequency lower than that of the ribbon type transducer. However, there is such a drawback that the magnetic circuit is complicated, it is rather difficult to assemble the parts, and a magnet with great magnetomotive force is required.